A wideband code division multiple access (W-CDMA) base transceiver station (BTS) may transmit a signal, which may be reflected, and/or attenuated by various obstacles and surrounding objects. As a result, various copies of the transmitted signal, at various power levels, may be received at the mobile station comprising different time offsets. FIG. 1a is a diagram exemplary of copies of a transmitted signal that may be received by a mobile receiver. Referring to FIG. 1a, there is shown a transmitter 2, a receiver 4, the group of transmitted signals 6, the group of received signals 8, and individual copies of the transmitted signals 10, 12, and 14. The plurality of signals 10, 12, and 14, received by a receiver 104 may be referred to as multipath signals while each signal in the multipath may be referred to as an individual distinct path signal. An individual distinct path signal may correspond to a path. A rake receiver may be deployed to demodulate individual distinct path signals, with each of a plurality of fingers assigned to track and demodulate one component of the multipath. The output of the fingers may then be combined and further demodulated and decoded. The fingers may be adapted to receive and process as much of the received signal energy as practicable.
A considerable part of receiver design may involve managing the rake receiver fingers. A functional block known in the art as a “searcher” may be adapted to locating new individual distinct path signals and to allocating rake receiver fingers to the new individual distinct path signals. The searcher may detect a path based on the amount of energy contained in a signal, identify that path if it carries user's data, and subsequently monitor the detected path. Once the detected signal energy in a path is above a given threshold, a finger in the rake receiver may be assigned to the path and the signal energy level constantly monitored.
However, partitioning the received signal into several fingers, each of which may process and exploit energy in a single individual distinct path, may have limitations. For example, a group of individual distinct path signals may rarely be characterized by a few discrete times of arrival. The result may be a method and apparatus, for example, a rake receiver, that may be inefficient at exploiting the power in received signals. In addition, utilizing this method may incur high processing overhead in managing the fingers. The total amount of time that transpires comprising the time to identify a path, to the time required to assign a finger, and to the time that the signal energy may be exploited, may account for 20-30% of the path life span. Once a finger is assigned to a path, detected energy on the path may be continuously monitored. However by the time that the finger has been assigned, the path energy may be diminished, while energy may rise at a different time of arrival. This may result in the rake receiver constantly searching for new paths, and performing finger de-allocation/allocation cycles. A finger that is allocated to a path with diminishing power may represent misused resources in the mobile terminal, which may in turn result lower performance of the mobile terminal.
Another limitation of a conventional rake receiver may be known in the art as finger merge or ‘fat’ finger. This is a phenomenon in which paths that are in close temporal vicinity of each may be may be assigned to separate fingers at the rake receiver. Finger merge may have negative implications for system performance for a variety of reasons.
The assignment of more than one finger to a single offset may be a waste of system resources, as the additional finger or fingers may be better deployed to receiving energy from another individual distinct path signal in the multipath, or to receive energy from a signal transmitted from another BTS. In addition, the combined power of the various fingers may often be used to control various system parameters, for example, power control. Without accounting for finger merge, a system may over-estimate the received power due the duplication of energy detected in the combiner, and thus over-compensate by lowering transmit power to a threshold level below that required for adequate communication.
Moreover, combining the output of merged fingers with the output of non-merged fingers may weight both the signal and noise of the merged finger output too heavily in relation to the non-merged finger output, which may result in inefficient exploitation of the received power. There may be a present need in the art to perform path search and resource allocation that reduces the searcher overhead by eliminating the need for micro-managing the fingers.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.